Currently, liquid crystal display devices with a wide viewing angle characteristic are used extensively in liquid crystal TVs and various other electronic devices. Most of those liquid crystal display devices operate in either the VA mode or the transverse electric field mode.
A VA mode liquid crystal display device includes a liquid crystal layer which is made of a nematic liquid crystal material with negative dielectric anisotropy. When no voltage is applied to that liquid crystal layer (and when a voltage that is lower than a threshold voltage is applied), liquid crystal molecules are aligned substantially perpendicularly (i.e., to define an angle of 85 to 90 degrees) to the surface of the substrate (more exactly, to the surface of the vertical alignment film). But when a voltage is applied to the liquid crystal layer, the liquid crystal molecules fall and get aligned substantially parallel to the surface of the substrate.
An MVA (multi-domain vertical alignment) mode liquid crystal display device, of which the viewing angle characteristic is superior to even that of the VA mode liquid crystal display device, is known (see Patent Document No. 1, for example). In the MVA mode device, by arranging two sets of linear alignment control structures (such as slits or ribs) that run in two mutually orthogonal directions, four liquid crystal domains (which will be simply referred to herein as “domains”), of which the representative director azimuths define an angle of 45 degrees with respect to the polarization axes (or transmission axes) of two polarizers that are arranged as crossed-Nicols, are formed between the alignment control structures. Supposing the three o'clock direction defines an azimuth angle of 0 degrees and the counterclockwise direction is positive if the display screen is compared to the face of a clock, the directors of those four liquid crystal domains define azimuth angles of 45, 135, 225 and 315 degrees, respectively. Such a structure in which four domains are formed in each single pixel is sometimes called a “quadruple alignment structure” or simply “4D structure”. Among those MVA mode liquid crystal display devices, ones that use slits that have been cut through electrodes as the alignment control structures to be provided on the surface of two substrates that face each other with a liquid crystal layer interposed between them are sometimes called a PVA (patterned vertical alignment) mode device.
The applicant of the present application developed and mass-produced VATN (vertical alignment twisted nematic) mode liquid crystal display devices.
FIG. 10(a) illustrates the structure of a pixel Px of a VATN mode liquid crystal display device produced by the applicant of the present application. As shown in FIG. 10(a), the pixel Pc has four kinds of domains. The azimuth angle of each of the directors that characterize these four kinds of domains defines an angle of 45 degrees with respect to the polarization axes (i.e., transmission axes) of two polarizers that are arranged as crossed Nicols. The director of each of these domains corresponds to the tilt direction of liquid crystal molecules (which are illustrated as circular cones in FIGS. 10(a) and 10(b)) that are located around the middle of the thickness of the liquid crystal layer. If the two polarization axes are respectively arranged in the vertical and horizontal directions on the display screen and if the display screen is compared to the face of a clock, supposing the three o'clock direction defines an azimuth angle of 0 degrees and the counterclockwise direction is positive, the directors of those four kinds of domains define azimuth angles of 135, 225, 315 and 45 degrees, respectively, as shown in FIG. 10(b). These four kinds of domains will be referred to herein as A+, B+, A− and B−, respectively (see FIG. 10(c)). A VATN mode liquid crystal display device, of which each pixel has those four kinds of domains, is produced by subjecting its alignment film to an optical alignment treatment as disclosed in Patent Documents Nos. 2 and 3, for example. Such a technique for improving the viewing angle characteristic by forming multiple different kinds of domains, of which the directors define mutually different alignment directions, in a single pixel is often called an “alignment division” technique.
In the liquid crystal display device shown in FIG. 10(a), each pixel Px has two subpixels SP1 and SP2, which exhibit mutually different luminances at least when a certain half-scale tone is displayed. That is to say, when the pixel Px displays a certain half-scale tone (which may be any grayscale level but grayscale level 0 for black display and grayscale level 255 for white display in the case of a 256 grayscale display), one of the two subpixels SP1 and SP2 exhibits a luminance corresponding to a higher grayscale level (>46) than that half scale tone (which may be level 46) and the other subpixel exhibits a luminance corresponding to a lower grayscale level (<46) than the half scale tone. Such a state is realized by applying mutually different voltages to the respective liquid crystal layers of the two subpixels SP1 and SP2 (see Patent Document No. 4, for example). The two subpixels SP1 and SP2 have mutually different γ characteristics (i.e., the grayscale dependences of the display luminance), and the γ characteristic of the pixel Px is represented as the average of the two γ characteristics. That is why the viewing angle dependence of the γ characteristic can be reduced. Such a technique for reducing the viewing angle dependence of the γ characteristic by defining multiple subpixels (such as SP1 and SP2), of which the liquid crystal layers may be supplied with mutually different voltages, in a single pixel Px in this manner is often called a “pixel division” technique. For example, the pixel Px shown in FIG. 10(a) is divided into two subpixels by the pixel division technique and each of those two subpixels SP1 and SP2 is further divided into four domains by the alignment division technique. A lot of specific methods for realizing the pixel division are known and have their own advantages and disadvantages. As for a normally black mode liquid crystal display device such as a VA mode device, it is recommended that the device is configured to widen the luminance difference between the subpixels at low grayscales (see Patent Document No. 4, for example).
In this description, a term “color display pixel” will be used along with the terms “pixel” and “subpixel”. The minimum unit of color display in a direct viewing color liquid crystal display device is called a “color display pixel”. Such a color display pixel consists of multiple pixels that represent multiple different primary colors. For instance, in the example illustrated in FIG. 10(a), the color display pixel Pc is made up of R, G and B pixels. Recently, in order to broaden the range of colors that can be reproduced by a liquid crystal display device (which is called a “color reproduction range”), techniques for increasing the number of primary colors for use to perform a display operation have been proposed. For example, Patent Document No. 5 discloses a liquid crystal display device that uses a color display pixel including not only red (R), green (G) and blue (B) pixels but also at least one more color pixel (which may be a yellow (Ye) pixel, a cyan (Cy) pixel, a magenta (Mg) pixel or a white pixel). If a white pixel is added, then the color reproduction range cannot be broadened but the display luminance can be increased. Examples of known pixel (or primary color) arrangement patterns include not only the vertical striped arrangement shown in FIG. 10 but also a horizontal striped arrangement, a pentile arrangement, and a delta arrangement. Meanwhile, a liquid crystal display device for use in a display device that conducts a color display operation by field sequential driving and a monochrome display liquid crystal display device do not include any color filters and do not have any color display pixels.